The present disclosure relates to semiconductor device fabrication, and more particularly to a method to prevent undesired undercutting of dielectric fin caps during fin field effect transistor (FinFET) fabrication in a replacement gate scheme.
Fin field effect transistors (FinFETs) are one of the most promising candidates for scaling beyond 32 nm node. A FinFET typically includes a source region and a drain region interconnected by fins which serve as a channel region of the device and a gate that surrounds at least a portion of each of the fins between the source region and the drain region. Epitaxial deposition is typically used to form the source region and the drain region. Current epitaxial deposition processes typically require a pre-clean process to remove surface defects or contaminations residing on the starting surface of a semiconductor substrate targeted for deposition. The conventional pre-clean process generally uses either an aqueous solution of hydrogen fluoride (HF), or a gas phase HF to remove the surface defects or contaminations.
In fabricating FinFETs, it is desirable to have a relatively thick oxide (or other dielectric material) cap on top of each fin to prevent damages to the fins during device processing especially during gate spacer formation which typically involves a reactive ion etch (RIE). However, when the epitaxial deposition is applied for fabricating a source region and a drain region, portions of the thick oxide cap underneath the gate spacers are susceptible to undercut during the extensive epitaxy pre-clean process. The undercut leads to epitaxy creeping under the spacer and excessive dopant diffusion at the top of the fins. As such, a method to prevent undercutting of the oxide capping layer during the epitaxy pre-clean process is needed.